As the cover stock of two-piece golf balls and some wound golf balls, ionomer resins in the form of ethylene-(meth)acrylic acid copolymers have been widely used and accepted because of their long-lasting impact resistance and cut resistance. Since the ionomer resins provide hard hitting feel and high hardness as compared with the balata rubber conventionally used as the cover stock, ionomer covered golf balls are said difficult to impart a desired quantity of spin and inferior to control on iron shots.
For improvements in these respects, Sullivan, U.S. Pat. No. 4,884,814 or JP-A 308577/1989 proposes to blend a hard ionomer resin in the form of a zinc or sodium salt of an ethylene/(meth)acrylic acid copolymer having a certain spectrum of physical properties with a specific amount of a soft ionomer resin in the form of a zinc or sodium salt of an ethylene/(meth)acrylic acid/(meth)acrylate terpolymer. The soft/hard ionomer blend is used as a golf ball cover. This is a quite effective technique for improving the hitting feel and control of golf balls using a conventional ionomer resin in the form of an ethylene-(meth)acrylic acid copolymer as the cover.
Nevertheless, the golf ball cover made of the above-mentioned blend of soft and hard ionomers of zinc or sodium salt type has several problems. Despite improvements in hitting feel and control, the cover is softer and allows the ball to receive more spin on iron shots, which means the increased frictional force between the club face and the cover. There is a likelihood that the ball surface be marred or fluffed as a consequence of iron shots because the cover surface can be scraped by grooves across the iron club face.
Due to the reduced hardness, the ionomer cover itself is reduced in restitution, resulting in a ball with a substantial loss of restitution.